This type of communication forms part of the field of application known by the term RFID (Radio Frequency IDentification). It is used for example for contactless badges or cards intended for authorizing access, or else identification tags stuck to objects to be identified.
In what follows, the card, the badge or the tag will be designated by the word “tag” or “RFID tag”, it being understood that this term does not limitingly designate a particular form of these objects.
Transmission is usually done by way of a radiofrequency magnetic field and it uses an inductive antenna forming part of the reader and an inductive antenna forming part of the tag. These antennas can consist of one or more turns of a conducting winding integrated into the tag. This winding is associated with electronic circuits intended for carrying out functions of frequency tuning, damping and impedance matching, and the assembly consisting of the winding and these auxiliary components will be called an inductive antenna. It is mainly the voltages induced in the windings by the high-frequency magnetic field which serve to transport information from the reader to the tag and vice versa. In addition to the inductive antenna serving for information exchange and optionally for energy reception, the tag generally comprises an integrated circuit microchip which handles the function of analysing the interrogation data and the production of the response.
In these systems, the reader generally dispatches a radiofrequency carrier, modulated in accordance with a communication protocol so as to constitute a radiofrequency interrogation signal; a typical frequency in the case of tags with inductive antenna is 13.56 MHz, modulated in amplitude or in phase or in frequency and it is this frequency that will be mentioned in the examples which follow. The reader then waits for a response; if a tag is present in the geographical zone close to the reader and if the interrogation signal involves a response therefrom, the tag reacts and emits an item of information in return towards the reader. The response may be the emission of a simple digital identification, or else it may be more complex.
The tag may be devoid of any autonomous power supply source, and in this case the energy which enables it to emit its response is provided in an inductive manner by the radiofrequency magnetic field produced by the antenna of the reader via the antenna of the tag; the energy can even be provided by the modulated radiofrequency magnetic field during the interrogation signal communication times, for communication applications at short distances (up to a few tens of centimeters).
The inductive antenna of the reader preferably plays both a role of signal emission antenna and a role of reception antenna for detecting an electromagnetic field modulation induced by the response of the tag. The response of the tag is generally established in the form of a modulation of impedance of its antenna; because of the electromagnetic coupling of the antenna of the tag and of the antenna of the reader when the tag is in a reactive zone (distance less than a twentieth of the wavelength, where the magnetic field is mainly of reactive rather than radiative type), this modulation gives rise to a modification of the impedance of the antenna of the reader, such as it is seen by the reader's circuits connected to this antenna; it is this impedance modification which allows the detection and then the analysis of the response.
The reader modulates its signal with carrier frequency Fp for a duration T so as to constitute a recognizable interrogation signal destined for the tag; there follows a second duration T′ where the signal with carrier frequency Fp of the reader is not modulated, so as to receive a response from a tag, if one is present.
Various systems operating along these principles have already been proposed in the past, for aiding not only the identification of objects (by way of an RFID tag stuck to the object) but also the locating of the objects in space.
In patent publication FR 2 891 980, RFID tags are at known positions and make it possible to locate a reader.
In patent publication FR 2 907 950, a mobile reader moves and identifies objects furnished with a tag when it passes in proximity to these objects. It knows its own position at any moment by virtue of a satellite positioning system and deduces therefrom the position of the objects encountered.
In the publication US 2008/0278289, beacons produce a magnetic field. A tag passing in proximity detects the variable levels of magnetic field received from the various beacons, dependent on the relative position of the tag with respect to the beacons; these levels are dispatched to a reader which deduces therefrom a position of the tag.
In patent publication US 2009/0085741, an RFID reader is located with respect to tags with known positions.
In patent publication WO 2008/45075, the position of an object provided with an RFID tag is detected with respect to an array of fixed antennas constituting as many fixed readers each capable of individually detecting the presence of a mobile tag. The reading circuits proper are grouped into a single reading circuit linked electrically to each of the fixed antennas by as many individual reading pathways (consisting of direct electrical connections) as there are fixed antennas.
Finally, in patent publication WO 2009/034526, there is a radiofrequency antenna matrix constituting an antenna with configurable radiation pattern making it possible to focus the RFID link in a particular direction, with the aim of following the three-dimensional displacements of an object carrying an RFID tag.